Determination of the time delay and/or power of a multicarrier signal

ABSTRACT

The invention relates to a method of determining the time shift and/or the power of a received signal incorporating a reference sequence ( 3 ) and transmitted by multicarrier modulation on a set (M) of carriers spaced from each other by a distance nΔf where n is a natural integer and Δf is a constant. An inverse Fourier transform is applied to the phase variations between components of the signal carried by carriers separated by kΔf for consecutive values of k in order to obtain the impulse response (RI) of the signal and the time shift τ 0  and/or the power of the received signal is/are determined from the impulse response by observation of the highest level amplitude peak.

FIELD OF THE INVENTION

The present invention relates to a method of determining the time shiftand/or the power of a received signal transmitted between one or moreuser terminals and a base station or between a base station and one ormore user terminals.

The invention also relates to a receiver circuit for implementing themethod and to a receiver device.

DESCRIPTION OF THE RELATED ART

The invention applies to the field of transmitting signals bymulticarrier orthogonal frequency division multiplex (OFDM) modulationcombined with the multicarrier code division multiple access (MC-CDMA)technique, with the orthogonal frequency division multiple access(OFDMA) technique, or with the time division multiple access (TDMA)technique.

In the signal transmission field, it is always necessary to synchronizereceived signals at the receiver in order to be able to process them.

Different time synchronization techniques have already been proposed indifferent contexts.

In particular, the standards embodied in the ETSI documents “RadioBroadcasting Systems: Digital Audio Broadcasting (DAB) to mobile,portable and fixed receivers” (April 2000, reference En 300 401 V1.3.1)and “Digital Video Broadcasting (DVB); Framing structure, channel codingand modulation for digital terrestrial television” (July 1999, referenceEN 300 744 V1.2.1) propose relatively complex time modulationtechniques.

In the particular context of the DVB-RCT standard, which relates toterrestrial interactive digital television, it is particularly importantto use a synchronization method that does not require too muchcomputation power of the digital signal processing means becauseexpansion of the terrestrial network must not be impeded by the cost ofreceivers.

In an OFDMA system, each user transmits a reference sequence on a set ofcarriers distributed at random, each carrier set being specific to oneuser. This is known in the art.

There is also a reference sequence in an MC-CDMA system, in which one ormore users forming a user group transmit(s) on the same set of carriers,different users using different respective reference sequences, which inthis context are referred to as “codes”.

The invention is based on the existence of this kind of referencesequence in the signals transmitted and applies to any system fortransmitting by multicarrier modulation signals that incorporate areference sequence.

SUMMARY OF THE INVENTION

The present invention consists in a method of determining the time shiftand/or the power of a received signal incorporating a reference sequenceand transmitted by multicarrier modulation on a set of carriers spacedfrom each other by a distance nΔf where n is a natural integer and Δf isa constant, which method is characterized in that it consists in:

-   -   determining a phase variation representative of the phase        variation between two components of the reference sequence of        the received signal on two carriers separated by kΔf, where k is        an integer, for at least two consecutive values of k,    -   applying an inverse Fourier transform to the representative        phase variations obtained in order to obtain the impulse        response of the signal, and    -   determining the time shift τ0 and/or the power of the received        signal from the impulse response by observation of the highest        level amplitude peak.

Thus the invention consists in searching the carriers conveying thesignal for the phase variation that corresponds to the time shift of thesignal.

The phase variations between the carriers are determined by differentialdemodulation and the phase variations determined in this way are relatedto families of carrier pairs, each family combining carrier pairs inwhich the two carriers are separated by the same integer multiple of aunit intercarrier offset Δf between the carriers used.

Having reordered the phase variations obtained from increasing ordecreasing but contiguous multiples of the unit intercarrier offset Δf,an inverse Fourier transform may be applied to obtain the impulseresponse of the signal and to deduce therefrom, merely from theamplitude peak supplied by the impulse response, either the time shiftor the power of the received signal.

The method of the invention determines not only the time shift of thesignal but also, by measuring the height of the resulting amplitudepeak, it determines power information characteristic of the receivedsignal.

In this way it is possible to control the power of users seeking tocohabit the same frequency band by applying feedback to theirtransmitters.

The effect of power control is that the receiver receives at the samepower level the signals transmitted by all users. Power controltherefore limits the problem of dazzle by preventing a user transmittingat a high power and therefore interfering strongly with signalstransmitted by other users.

As will become apparent on reading the examples described below, thecomplexity of the method of the invention depends essentially on twoparameters, namely the number Nfft of values of the integer k for whicha representative phase variation is determined and the method ofdetermining the representative phase variation for each value of k,given that there are several pairs of carriers spaced by the offset kΔf.

In one particular implementation of the invention, the representativephase variation for a value of k is determined by calculating theaverage of the phase variations of a plurality of pairs of components ofthe received reference sequence conveyed by carriers separated by kΔf.

In this case, the number of terms used to calculate the average is animportant parameter that conditions the complexity of the method of theinvention.

The method of the invention is easier to put into practice if the set ofcarriers used to transmit the signal enables pairs of carriers to beconstituted in which the two carriers of a pair are separated by anoffset kΔf with k varying from 1 to Nfft.

In the context of the DVB-RCT standard, each set of carriers (alsoreferred to as a “subchannel”) usable by a group of users is made up of145 carriers that can be grouped in pairs covering all values of k from1 to 256.

If, of all the carriers of a subchannel, it is not possible to selecttwo that are separated by the offset kΔf, the invention proposes eitherusing the value zero as the representative phase variation for thisvalue of k or else using the average of the representative phasevariations obtained for the adjacent values of k.

This is the case in particular in the DVB-RCT standard for the followingvalues of k between 1 and 512, for subchannel 0 of the 1K mode: 406,435, 484 and 493.

In a different implementation of the method of the invention, for eachvalue of the integer k, the phase variation between two components ofthe received signal carried by first and second carriers separated bykΔf is calculated first, after which the representative phase variationis calculated by multiplying the phase variation obtained by the productof the reference sequence component on the first carrier and theconjugate of the reference sequence component on the second carrier.

Apart from the fact that this implementation reduces the number ofoperations to be effected by omitting calculation of the average of thephase variations, it entails multiplication by a product known inadvance and whose values may be prerecorded.

In a particularly advantageous variant of this implementation, appliedto transmitting signals by using regularly distributed carriers, foreach value of the integer k, the phase variation is calculated between abasic component of the signal received on a basic carrier, chosen assuch from the carriers used, and a component obtained by time-delaying ktimes the basic component with an intercarrier spacing Δf.

In other words, account is taken of the fact that the carriers areregularly distributed in order to consider only the first k pairs ofcarriers formed by the basic carrier and the first k carriers of thespectrum.

In the case of regularly distributed carriers, there is no benefit inconsidering all the available carriers to determine the requiredconsecutive spacings.

The invention also consists in a receiver circuit adapted to determinethe time shift and/or the power of a received signal incorporating areference sequence and transmitted by multicarrier modulation on a setof carriers spaced from each other by a distance nΔf where n is anatural integer and Δf is a constant, which circuit is characterized inthat it comprises:

-   -   means for determining a phase variation representative of the        phase variation between two components of the reference sequence        of the received signal on two carriers separated by kΔf, where k        is an integer, for at least two consecutive values of k,    -   means for applying an inverse Fourier transform to the        representative phase variations obtained in order to obtain the        impulse response of the signal, and    -   means for determining the time shift and/or the power of the        received signal from the impulse response by observation of the        highest level amplitude peak.

The invention further consists in a signal receiver incorporating themeans referred to hereinabove.

To facilitate an understanding of the invention, implementations of theinvention are described below by way of illustrative and nonlimitingexample.

The examples used to explain the invention come from the context of timesynchronization and power control based on the MC-CDMA technique as usedin the draft DVB-RCT standard for terrestrial interactive digitaltelevision.

The person skilled in the art will know how to identify the use that ismade in these examples of the reference sequence (referred to herein asa code) and how to transpose the invention to other standards alsoemploying a reference sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of an MC-CDMA transmission system using animplementation of the method of the invention,

FIG. 2 shows seven carriers used to transmit a signal,

FIG. 3 is analogous to FIG. 2 and shows the offsets between carriersconsidered two by two,

FIG. 4 is a block diagram of an MC-CDMA receiver system using adifferent implementation of the method of the invention,

FIG. 5 is a block diagram of an MC-CDMA receiver system constituting avariant of the FIG. 4 system applied when the carriers conveying thereference sequence are regularly distributed,

FIG. 6 is a graph of the impulse response obtained using the method ofthe invention and considering only one user, and

FIG. 7 is analogous to FIG. 6 but considers two users.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an uplink transmission system whichcomprises a group 1 of k+1 user terminals and a base station 2 thatserves as a demodulator for retrieving the signals sent by each userfrom all of the received signals S₀, . . . , S_(k), which togetherconstitute an overall received signal S.

In his terminal U₀, . . . , U_(k) each user sends a code 0, k referenced3 that is specific to the terminal on a set M of subcarriers selectedduring a step 4.

FIG. 1 represents only the situation in which k+1 users send on all Mcarriers.

The k+1 users may send their own reference sequence referred to hereinas a code, simultaneously on the same set M of carriers, whilst at thesame time k′+1 other users (this group is not shown) may sendsimultaneously on another set M′ of carriers.

Each set of carriers is called a subchannel. The first two subchannelsof the 1K mode of the DVB-RCT standard are referred to below by way ofexample.

Subchannel 0:

8 10 13 14 24 37 42 43 51 53 66 72 75 80 82 95 104 109 111 112 124 133134 138 141 151 162 163 167 170 176 180 191 192 199 205 209 221 223 228234 238 250 252 258 263 267 281 287 289 292 309 310 316 318 335 338 339345 347 348 364 367 374 376 377 392 393 396 405 406 411 422 423 426 436441 451 452 457 465 470 472 480 486 499 501 506 509 515 527 528 530 535544 556 559 563 564 573 585 588 592 593 604 614 621 622 633 637 643 648650 662 666 671 677 679 691 695 698 700 706 720 724 727 729 735 736 753756 758 764 765 768 785 787 794 797 798 814 823 826 827 837Subchannel 1:6 17 18 22 25 31 35 46 47 54 60 64 76 78 83 89 93 105 107 113 118 122136 142 144 147 164 165 171 173 190 193 194 200 202 203 219 222 229 231232 247 248 251 260 261 266 276 277 280 290 295 305 306 311 319 324 326334 340 353 355 360 363 608 611 613 619 620 623 640 642 649 652 653 669678 681 682 692 705 707 710 711 721 734 739 740 748 750 763 769 772 777779 792 801 806 808 809 821 830 831 835 838

Each subchannel is made up of 145 carriers. The sets M and M′ ofsubcarriers yield all the values of k from 1 to 256 (for k=1 to 512,four values of k must be determined by interpolation).

The explanation of the method of determining the time shift considersseven subcarriers P1, P3, P4, P8, P10, P15 and P23 of the OFDM spectrum,as shown in FIG. 2.

The signals to be sent are modulated by a modulator 5 using an inverseFourier transform and then sent.

Since the various users send asynchronously to the base station, eachuser is received with a specific time shift, as schematized in 6.

If x_(j)(t) is the signal sent by the user j on an OFDM symbol ofduration Ts, in which the spectrum of the various carriers results fromrectangular shaping filtering in the time domain, then the expressionfor x_(j)(t) is:

$\begin{matrix}{{{xj}(t)} = {\sum\limits_{k \in M}{\mathcal{R}\;{e\left( {C_{n\; k}^{j}{\exp\left( {2\;{\mathbb{i}}\;\pi\; f\;{k\left( {t + {\tau j}} \right)}} \right)}} \right.}}}} & (1)\end{matrix}$

In equation (1), C^(j) _(nk) corresponds to the chip of index n_(k) (achip is a fraction of the code sent) of the code of the user jmodulating the carrier k (also referred to as a component of thereference sequence on the carrier k) and τj is the time-delay with whichthe signal from the user j reaches the base station.

In the base station 2, the global received signal is transposed into thefrequency domain by a demodulator 7 using a direct Fourier transform.

The method of determining the time shift in the base station isexplained below.

The following description considers only one user whose sent signal isshifted by τ0.

The received signal y(t) corresponds to the signal as sent affected by adisturbance H_(k) linked to attenuation and phase rotation introduced oneach subcarrier by the channel.

The expression for the received signal y(t) is:

$\begin{matrix}{{y\;(t)} = {\sum\limits_{k \in M}{\mathcal{R}\;{e\left( {H\; k\; C_{n\; k}{\exp\left( {2\;{\mathbb{i}}\;\pi\; f\;{k\left( {t + {\tau 0}} \right)}} \right)}} \right.}}}} & (2)\end{matrix}$

After application of the Fourier transform during reception in step 7,the signal received on each of the subcarriers k has the value:Y _(k) =H _(k) C _(nk) exp(−2iπf _(k)τ0)  (3)

The carriers of the set M are extracted by a unit 8 after which amultiplier 9 effects the complex conjugate multiplication of the signalreceived on each subcarrier and the chip of the code specific to theuser and known to the receiver, after which a differential demodulator10 calculates a representative phase variation.

The determination of the representative phase variation relies on adifferential frequency domain method that yields the average phasevariation between different pairs of carriers from the set M separatedby a distance kΔf with k=1, . . . , Nfft.

Δf corresponds to the intercarrier spacing, which is equal to 1/t_(s)where t_(s) is the usable duration of the OFDM symbol.

Nfft corresponds to a parametrizable value that depends on the set M andon the envisaged complexity of the system.

FIG. 3 sets out all the carrier pairs corresponding to different valuesof k.

The phase variation between two signals separated by Δf is obtained bydifferentially demodulating the signal conveyed by the carrier P4 andthe signal conveyed by the carrier P3.

The operation effected is to form the product P4.P3*, where P3* is theconjugate of P3.

In this way it is possible to determine all of the values relating toeach value of k.

If there are more than one pair of carriers corresponding to the samevalue of k, then the average of the differential demodulations obtainedfor that value of k may be used.

In the FIG. 3 example, the offset 2Δf is obtained when considering thecarrier pairs {P1, P3} and {P8, P10}. In this case, it is possible toaverage the differential demodulations, as follows:

$\frac{{P3P1}*{+ {P10P8}}*}{2}$

The object of this averaging is to obtain a better estimate of the phasevariation between two carriers separated by kΔf, averaging the variousforms of interference encountered (propagation channel fluctuations,multiple access interference, noise, etc.). This value is taken as therepresentative phase variation for the value k.

If the set M of carriers cannot provide all the required values of k, itis possible either to consider that the differential modulation valuecorresponding to this value of k is zero or to carry out aninterpolation operation by considering the indices of the values of knearest the unlisted value of k for which a differential demodulationmay be defined. The latter option gives better results.

By applying this differential demodulation to the first eight carrierindices of subchannel 0 of the 1K mode, the following set of values isobtained at the output of each differential demodulator 10:

$\begin{matrix}\begin{matrix}{{C3C}_{3}^{*}{H14}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f14}\;{\tau 0}} \right)}\left( {{C2C}_{2}^{*}{H13}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f13}\;{\tau 0}} \right)}} \right)* +} \\{{C7C}_{7}^{*}{H43}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f43}\;{\tau 0}} \right)}} \\{{\left( {{C6C}_{6}^{*}{H42}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f42}\;{\tau 0}} \right)}} \right)*};{{C1C}_{1}^{*}{H10}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f10}\;{\tau 0}} \right)}}} \\{{\left( {{C0C}_{0}^{*}{H8}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f8}\;{\tau 0}} \right)}} \right)*};} \\{{{C2C}_{2}^{*}{H13}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f13}\;{\tau 0}} \right)}\left( {{C1C}_{1}^{*}{H10}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f10}\;{\tau 0}} \right)}} \right)*};} \\{{C3C}_{3}^{*}{H14}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f14}\;{\tau 0}} \right)}} \\{{\left( {{C1C}_{1}^{*}{H10}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f10}\;{\tau 0}} \right)}} \right)*};{{C2C}_{2}^{*}{H13}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f13}\;{\tau 0}} \right)}}} \\{{{\left( {{C0C}_{0}^{*}{H8}\;{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\;{f8}\;{\tau 0}} \right)}} \right)*};}\;}\end{matrix} & (4)\end{matrix}$

which yields the following set of values (in increasing order of kΔf):(H₁₄H₁₃*+H₄₃H₄₂*)exp(−2iπΔfτ0); H₁₀H₈*exp(−4iπΔfτ0);H₁₃H₁₀*exp(−6iπΔfτ0); H₁₄H₁₀*exp(−8iπΔfτ0); H₁₃H₈*exp(−10iπΔfτ0);  (5)

The set (5) of values may be generalized as follows:

$\begin{matrix}\begin{matrix}{{{\sum\limits_{\frac{k \in M}{{k - 1} \in M}}{HkHk}} - {1*{\exp\left( {{- 2}\;{\mathbb{i}}\;{\pi\Delta}\; f\;{\tau 0}} \right)}}};} \\{{{\sum\limits_{\frac{k \in M}{{k - 2} \in M}}{HkHk}} - {2*{\exp\left( {{- 4}\;{\mathbb{i}}\;{\pi\Delta}\; f\;{\tau 0}} \right)}}};\;\ldots\mspace{11mu};} \\{{\sum\limits_{\frac{k \in M}{{k - {Nff}} \in N}}{HkHk}} - {{Nfft}*{\exp\left( {{- 2}*{Nfft}*\;{\mathbb{i}}\;{\pi\Delta}\; f\;{\tau 0}} \right)}}}\end{matrix} & (6)\end{matrix}$

What is required from all the terms of the expression (6) is to averagethe channel fluctuations over all the carriers separated by kΔf in orderto obtain a better estimate of the impulse response of the propagationchannel and therefore of the received propagation time-delay. It istherefore necessary to normalize each term by the number of sumsconstituting it:

$\sum\limits_{\frac{k \in M}{k^{\prime} \in M}}{HkHk}^{\prime*}$

For a single-path channel (Hk=Hk′*=1), each term of the expression (4)becomes:

$\begin{matrix}{\sum\limits_{k = 1}^{Nfft}{\exp\left( {{- 2}\;{\mathbb{i}}\;\pi\; k\;\Delta\; f\;{\tau 0}} \right)}} & (7)\end{matrix}$

A modulator 11 applies an inverse Fourier transform to the equation (7)to obtain the impulse response RI of the channel, yielding:

This method yields a Dirac centered on τ0 relative to the time shift ofthe signal sent.

This determines the time shift.

It is seen that, quite apart from the performance achieved by the methodof the invention, it is possible to vary its complexity.

The complexity of the method depends on the size of the inverse Fouriertransform needed to obtain the impulse response of the channel and onthe number of averaging points (Pt_avg) for each value of k used todetermine the representative phase variation for each offset kΔf.

Equation (9) gives the number of complex multiplications the demodulatorhas to perform to implement the proposed method:

$\begin{matrix}\begin{matrix}{\underset{\underset{{global}{\mspace{11mu}\;}{reception}\mspace{14mu}{FFT}}{︸}}{\frac{Nfft\_ glob}{2}*\log\; 2\frac{Nfft\_ glob}{2}} + \underset{\underset{{multiplications}\mspace{11mu}{by}\mspace{14mu}{codes}}{︸}}{K*N} + \underset{\underset{{differential}\mspace{14mu}{method}}{︸}}{K*{Nfft}*{pt\_ avg}} +} \\{\underset{\underset{{divisions}\mspace{14mu}{for}\mspace{14mu}{average}}{︸}}{K*{Nfft}} + \underset{\underset{{transposition}\mspace{14mu}{to}\mspace{14mu}{time}\mspace{14mu}{domain}\mspace{14mu}{by}\mspace{14mu}{IFFT}}{︸}}{K*\frac{Nfft}{2}\log\; 2\frac{Nfft}{2}}}\end{matrix} & (9)\end{matrix}$

In equation (9), Nfft_glob is the number of points of the Fouriertransform in reception, K is the total number of possible codesequences, N is the size of the code sent, Nfft is the number of pointsover which the inverse Fourier transform is to be applied to determinethe impulse response of each channel, and Pt_avg is the number of pointsused for averaging to obtain the representative phase variation for eachvalue of kΔf for k=1, . . . , Nfft.

To obtain a good representation of the impulse response of the channel,it is necessary to vary the values of Nfft and Pt_avg allowing for thecomplexity that they generate.

The values taken by these two constants depend on the system parameters(number of carriers constituting the set M, values of the indices of thecarriers of the set M, etc.) and must be adjusted according to thenature of the interference encountered (fluctuations of the propagationchannel, multiple access interference, noise, etc.).

It is found that one factor contributing to the complexity of the methodof the invention is the calculation of an average phase variation toobtain a representative phase variation for a given value of k.

The method may be simplified by not using any such average.

Accordingly, as may be seen in FIG. 4, after application by thedemodulator 7 of the direct Fourier transform in reception, theextractor 8 extracts all the carriers on which the code was sent (setM). FIG. 4 shows again, for the subcarriers, the indices of subchannel 0of the mode 1K of the DVB-RCT standard.

The multiplier 12 applies the differential demodulation to the set M,taking the carriers in their order of extraction (two successivecarriers being separated by kΔf for k=1, . . . , Nfft).

The operator 13 supplies to the multiplier 12 the conjugate of thecarrier previously extracted for multiplication with the carrier beingextracted.

Knowing the indices of the carriers of the set M that satisfy the kΔfoffsets, the relative differential demodulation values are ordered inthe memory 14 in increasing or decreasing order of k (one value for eachvalue of k).

The multiplier 15 then multiplies each of the k values obtained by thisdifferential demodulation by the term C_(i,p)C_(j′,p)* (for i−i′=k)taken from a file specific to the set M and to the user.

At this stage the Nfft samples necessary for application by themodulator 16 of the inverse Fourier transform to obtain the impulseresponse RI of the propagation channel are available.

The time shift and the amplitude variation may be determined in order toadjust time synchronization and user power control.

In this case, the number of complex multiplications for a user is:

$\begin{matrix}\begin{matrix}{\underset{\underset{{global}\mspace{11mu}{reception}\mspace{14mu}{FFT}}{︸}}{\frac{Nfft\_ glob}{2}*\log\; 2\frac{Nfft\_ glob}{2}} + \underset{\underset{\begin{matrix}{{{differential}\mspace{14mu}{demodulation}} +} \\{{multiplication}\mspace{14mu}{by}\mspace{14mu} C_{j}C_{j + k}^{*}}\end{matrix}}{︸}}{2*{Nfft}} +} \\\underset{\underset{{transposition}\mspace{14mu}{to}\mspace{14mu}{time}\mspace{14mu}{domain}\mspace{14mu}{by}\mspace{14mu}{IFFT}}{︸}}{\frac{Nfft}{2}\log\; 2\frac{Nfft}{2}}\end{matrix} & (10)\end{matrix}$

In equation (11) below, N_(port) _(—) M corresponds to the number ofcarriers in the set M, which is naturally greater than the number ofpoints of the inverse Fourier transform to be calculated.

The method may be further simplified if the reference sequences are senton regularly distributed carriers. In this case, the intercarrierspacing is complied with and is by definition kΔf for the first Nfftcarriers of the spectrum.

It is then possible to limit the calculation to the differentialdemodulation for the first Nfft pairs of carriers, rather than for allthe pairs of carriers that may be formed for each value of k.

Accordingly, as shown in the FIG. 5 block diagram, after application ofthe receive Fourier transform by the demodulator 7, the differentialdemodulation is effected by first choosing a basic carrier Y_(p) (thefirst carrier of the spectrum, for example). This carrier is shifted ktimes (k=1, . . . , Nfft) at step 17 to effect Nfft differentialdemodulations by the multiplier 12 and the conjugation operation 13.

The multiplier 15 immediately multiplies each point obtained bydifferential demodulation by the term C_(i,p)C_(i)+_(k,p)* calculated inadvance and stored in a file (C_(i,p) corresponds to the i^(th) chipcarried by the carrier p).

It is then possible for a unit 16 to apply the inverse Fourier transformover Nfft points, as before, to obtain the impulse response of thepropagation channel and deduce therefrom the time shift and theamplitude variation that will be used for time synchronization and powercontrol.

In this case, the number of complex multiplications for a user is:

$\begin{matrix}\begin{matrix}{\underset{\underset{{global}{\mspace{11mu}\;}{reception}\mspace{14mu}{FFT}}{︸}}{\frac{Nfft\_ glob}{2}*\log\; 2\frac{Nfft\_ glob}{2}} + \underset{\underset{{differential}\mspace{14mu}{demodulation}}{︸}}{N_{port\_ M} - 1} +} \\{\underset{\underset{{multiplication}\mspace{14mu}{by}\mspace{14mu} C_{j}C_{j}^{*}}{︸}}{Nfft} + \underset{\underset{{transposition}\mspace{14mu}{to}\mspace{14mu}{time}\mspace{14mu}{domain}\mspace{14mu}{by}\mspace{14mu}{IFFT}}{︸}}{\frac{Nfft}{2}\log\; 2\frac{Nfft}{2}}}\end{matrix} & (11)\end{matrix}$

This is reflected in a very significant reduction in complexity.

FIG. 6 shows the impulse response obtained with the method of theinvention by calculating an inverse Fourier transform over 512 pointsand considering the situation in which a single user transmits with aunit power and with a relative time-delay of 0.3847T_(s), where T_(s) isthe usable duration of the symbol, i.e. the reciprocal of theintercarrier spacing Δf.

This time-delay corresponds to 394 samples for an inverse Fouriertransform covering 1024 points and to 197 samples for an inverse Fouriertransform covering 512 points.

Note that the transmission shift and the receive power level areobtained.

The dynamic range, i.e. the difference between the highest amplitudepeak and the secondary peaks, is 37 dB.

FIG. 7 shows the impulse response obtained in a situation where twousers transmit simultaneously, the second user being received with atime-delay of 0.1835T_(s).

Of course, the implementations that have just been described do notlimit the invention in any way and could be modified in any desirablemanner without departing from the scope of the invention.

1. A method of determining the time shift and/or power of a receivedsignal incorporating a reference sequence and transmitted bymulticarrier modulation on a set of carriers spaced from each other by adistance nΔf where n is a natural integer and Δf is a constant, whereinthe method comprises: determining, for each of at least two consecutivevalues of an integer k, a phase variation value associated with k andrepresentative of the phase variation between two components of thereference sequence of the received signal on two carriers separated bykΔf by differentially demodulating the signal conveyed by one of the twocarriers and the signal conveyed by the other of the two carriers,applying an inverse Fourier transform to a series formed by the phasevariation values ranked as a function of the associated k in order toobtain the impulse response of the signal, and determining the timeshift 0 and/or the power of the received signal from the impulseresponse by observation of the highest level amplitude peak.
 2. A methodaccording to claim 1, wherein at least one of the two components of thereference sequence of the received signal is obtained by multiplying acomponent of the signal received on a corresponding carrier by theconjugate of a component of the reference sequence on the correspondingcarrier.
 3. A method according to claim 2, wherein the representativephase variation for a value of k is determined by calculating theaverage of the phase variations of a plurality of pairs of components ofthe received reference sequence conveyed by carriers separated by kΔf.4. A method according to claim 1, wherein, if there is no pair ofcarriers separated by kΔf, the value zero is taken as the phasevariation for that value of k.
 5. A method according to claim 1,wherein, if there is no pair of carriers separated by kΔf, the averageof the representative phase variation obtained for the adjacent valuesof k is taken as the phase variation for that value of k.
 6. A methodaccording to claim 1, wherein, for each value of the integer k, thephase variation between two components of the received signal carried byfirst and second carriers separated by kΔf is calculated first, afterwhich the representative phase variation is determined by multiplyingthe phase variation obtained by the product of the component of thereference sequence on the first carrier and the conjugate of thecomponent of the reference sequence on the second carrier.
 7. A methodaccording to claim 6, wherein, for each value of the integer k, thephase variation between a basic component of the signal received on abasic carrier (y_(p)), chosen as such from the carriers used, and acomponent obtained by time-delaying k times the basic component with aninter-carrier spacing Δf is calculated.
 8. The method according to claim1 applied in orthogonal frequency division multiplex (OFDM) multicarriermodulation in combination with multicarrier-code division multipleaccess (MC-CDMA) technique.
 9. The method according to claim 1 appliedin multicarrier orthogonal frequency division multiplex (OFDM)modulation in combination with frequency division multiple access (FDMA)technique.
 10. The method according to claim 1 applied in multicarrierorthogonal frequency division multiplex (OFDM) modulation in combinationwith time division multiple access (TDMA) technique.
 11. A receivercircuit adapted to determine the time shift and/or the power of areceived signal incorporating a reference sequence and transmitted bymulticarrier modulation on a set of carriers spaced from each other by adistance nΔf where n is a natural integer and Δf is a constant, whereinthe circuit comprises: means for determining, for each of at least twoconsecutive values of an integer k, a phase variation value associatedwith k and representative of the phase variation between two componentsof the reference sequence of the received signal on two carriersseparated by kΔf by differentially demodulating the signal conveyed byone of the two carriers and the signal conveyed by the other of the twocarriers, means for applying an inverse Fourier transform to a seriesformed by the phase variation values ranked as a function of theassociated k in order to obtain the impulse response of the signal, andmeans for determining the time shift and/or the power of the receivedsignal from the impulse response by observation of the highest levelamplitude peak.
 12. A receiver of signals of the type incorporating areference sequence and transmitted by multicarrier modulation on a setof carriers spaced from each other by a distance nΔf where n is anatural integer and Δf is a constant, wherein the receiver comprises:means for determining, for each of at least two consecutive values of aninteger k, a phase variation value associated with k and representativeof the phase variation between two components of the reference sequenceof the received signal on two carriers separated by kΔf bydifferentially demodulating the signal conveyed by one of the twocarriers and the signal conveyed by the other of the two carriers, meansfor applying an inverse Fourier transform to a series formed by thephase variation values ranked as a function of the associated k in orderto obtain the impulse response of the signal, and means for determiningthe time shift and/or the power of the received signal from the impulseresponse by observation of the highest level amplitude peak.